The present invention generally relates to cellular and wireless communication. More specifically, the invention relates to a system and method for introducing a communication system in a low frequency reuse pattern.
Recently, there has been a trend in the telecommunication community to focus more and more on wireless packet data communication rather than circuit switched voice communication. With the tremendous increase of Internet users, it is believed that the packet switched communication will soon increase more and become larger than the circuit switched voice communication that today dominates, e.g., the cellular communication. Cellular communication system manufacturers and operators are therefore looking for solutions to integrate their circuit switched services with wireless packet switched services that can provide reliable and more spectrum efficient connections for packet switched users, e.g., Internet users. This trend has made different types of packet switched communication system evolutions flourish. One of the more well known packet switched cellular systems in the telecommunications community, is the extension of the present Global System for Mobile Communication (GSM), known as General Packet Radio Service (GPRS).
GPRS is a packet switched system that uses the same physical carrier structure as the present GSM cellular communication system and is designed to coexist and provide the same coverage as GSM. The GPRS radio interface is thus based on a TDMA (Time Division Multiple Access) structured system with 200 kHz carriers divided into eight timeslots with GMSK (Gaussian Minimum Shift Keying) modulation. The multiplexing is such that each timeslot can typically serve a number of users. One user can also be allocated more than one timeslot to increase its throughput of data over the air.
The GPRS specification includes a number of different coding schemes to be used dependent on the quality of the radio carrier. With GPRS, data rates well over 100 kbps will be possible.
There is also ongoing a development and standardization of a new air interface mode in GSM, which will affect both packet and circuit switched modes. This new air interface mode is called EDGE, Enhanced Data rates for Global Evolution. EDGE""s main features are new modulation and coding schemes for both packet switched and circuit switched data communication. In addition to the Gaussian Minimum Shift Keying (GMSK) modulation, an 8 symbol Phase Shift Keying (8PSK) modulation is introduced. This modulation can provide users with higher bit rates than GMSK in good radio environments.
A new technique called link quality control is introduced with EDGE. Link quality control is a functionality that allows adaptation in terms of coding and modulation with respect to present signal quality. In poor radio conditions, a robust coding and GMSK modulation is selected whereas in good radio conditions, a less robust coding and 8PSK modulation is used. GPRS (and the extensions thereof) also provides a backward error correction functionality in that it can request retransmissions of erroneously received blocks. This mechanism is called ARQ (Automatic Repeat reQuest) and is well known in the art.
The packet data mode with EDGE modulation is called EGPRS (Enhanced GPRS) and the circuit switched data mode is called ECSD, Enhanced Circuit Switched Data. Bitrates over 384 kbps will be possible with EDGE.
Recent development for another TDMA based cellular system, the cellular communication system compliant to the ANSI/136 standard, below referred to as TDMA/136, has been focused on a packet data system to be integrated with the TDMA/136 circuit switched mode.
This packet data system will also be based on the new EDGE technology as defined for the GPRS extension. It will then allow TDMA/136 operators to provide bit rates up to 384 kbps on 200 kHz carriers with GMSK and 8PSK modulation as defined for EGPRS.
This integration of TDMA/136 and EDGE, does not, however, come without a cost. The TDMA/136 carriers have a bandwidth of only 30 kHz, to be compared with EDGE carriers of 200 kHz. This means that operators that want to introduce EDGE, have to allocate 200 kHz for each EDGE carrier or, to put it in another way, to free up spectrum for each EDGE carrier corresponding to 7 already existing 30 kHz carriers. Since operators already today are using these 30 kHz carriers for circuit switched communications, there is a large interest that the initial deployment for EDGE in TDMA/136 systems should be made on as small a spectrum as possible.
Reuse patterns are used in cellular systems, such that one can reuse the same frequencies in different cells. Systems are usually planned such that a number of cells share a number of available channels. For example, in a {fraction (4/12)} frequency reuse, there are {fraction (4/12)} different cells that share a set of frequencies. Within these 4/12 cells, no frequency is used in more than one cell simultaneously. (The number 4 in xe2x80x9c{fraction (4/12)}xe2x80x9d denotes the number of base station sites involved in the 12 reuse. The {fraction (4/12)} denotation thus indicates that a base station site serves 3 cells.) These 12 cells then form what is referred to as a cluster. Clusters are then repeated, to provide coverage in a certain area.
Similarly in a 1/3 reuse, there are 3 different cells that share a set of frequencies. Within these 3 cells, no frequency is used in more than one cell simultaneously. Thus, the higher the reuse, the better the carrier to interference ratio for an exemplary condition. For lower reuse patterns, the carrier to interference ratio is lower, since the distance between two base stations transmitting on the same frequency is shorter. An exemplary ⅓ reuse is illustrated in FIG. 1.
GPRS channels typically have different levels of robustness depending on the type of logical channel being transmitted. A logical channel is defined by its information content and is transmitted on one or several physical channels, defined by the physical channel structure, e.g., a timeslot on a certain frequency. In a packet data system, reliance on retransmission possibilities can allow a quite high error rate which means that the reuse for user data traffic channels can be kept quite low. For example, a data traffic channel can be deployed in a ⅓ reuse whereas common control channels and broadcast channels are not robust enough to be allocated in a ⅓ reuse, since the same retransmission possibilities are not used for these types of logical channels. At least a {fraction (3/9)} or even a 4/12 reuse is recommended for packet data common control and broadcast channels.
Note that a {fraction (3/9)} reuse entails that at least nine 200 kHz carriers are needed (i.e., TDMA operators must provide at least 1.8 MHz of spectrum for an initial deployment). This is considered quite substantial in a TDMA system with 30 kHz carriers.
This fact has driven the TDMA community to find other solutions for initial deployment of a packet data system based on EDGE and GPRS. U.S. Pat. No. 6,438,115 entitled, xe2x80x9cHigh Speed Data Communication System and Methodxe2x80x9d, to Mazur et al., hereby incorporated by reference herein, teaches a method of combining TDMA/136 and the EGPRS mode of EDGE.
Briefly, the solution is to put requirements on the base station transmissions of the EDGE carriers. Base station transmissions of EDGE carriers should be time synchronised. It is then possible to allocate the control channels on different frequencies and different timeslots in different cells and thereby construct a higher reuse than what is possible by only considering frequencies. This solution is often referred to as EDGE Compact. In addition to the frequency reuse, a time reuse is introduced. For example, a certain base station transmits control signalling on a certain timeslot at a certain time and on a certain frequency, at which no other base station in the same control channel cluster (i.e., all cells where each physical channel carrying control signalling is used once and only once) is transmitting anything at all. This is repeated between a number of base stations, such that different time groups are formed. Further, to increase reliability of control channel detection in the mobile stations and base stations respectively, timeslots adjacent to each other do not both carry control channel information.
EDGE Compact provides the opportunity to introduce a higher reuse than that allowed by frequency repetition only. Thus, it will be possible to allow an initial deployment of a GPRS/EGPRS packet data system within a spectrum bandwidth much smaller than that otherwise limited by the reuse requirement for the control channels. In FIG. 4, a typical allocation for the control channels is illustrated. Therein, four different time groups are illustrated on a single frequency, i.e., a 4xc3x97time reuse is formed. In one cell, control information is transmitted in timeslot 1, (TS1), i.e., timegroup 1 (TG1), in certain GSM frames defined. Base stations transmitting control information on the same frequency but belonging to another time group, will not transmit at all during the frames that are used for control in base stations belonging to TG1. In another cell, control information is transmitted in TS3 (i.e., TG2), again in certain GSM frames. Base stations transmitting control information on the same frequency but belonging to another time group, will not transmit at all during the frames that are used for control in base stations belonging to time group 2. Similar reasoning applies for TS5 and TS7. Combining the time reuse with e.g., a ⅓ frequency reuse, it is possible to transmit control information in an effective {fraction (4/12)} reuse using only 3 frequencies. In FIG. 4, different types of control information or logical control channels have been indicated. In block B0, broadcast information is transmitted on a logical Broadcast Channel (BCCH) and, e.g., in block C8 logical Common Control Channels (CCCH) is transmitted (e.g., paging messages). The structure of the control channel is such that more blocks than those indicated can be allocated for broadcast or control. For example, if one more block is needed for CCCH, this can be allocated in physical block 2, on GSM frames 8-11. Allocation of 2-12 blocks is possible on a single timeslot. One broadcast information block and one common control block is always needed.
Further, to be able to find this control channel, a frequency correction burst and a synchronization burst is included in each 52 multiframe. A mobile will first search for the Frequency correction burst (located in GSM frame 25) and then it will know that following this, there will be a synchronization burst 26 GSM frames later, on the same timeslot. This synchronization burst will help the mobile station to identify the base station and to know where in the multiframe structure it is.
FIG. 3 illustrates an exemplary cell pattern that is formed of the reuse of time groups and frequencies combined. Note that in EDGE Compact, only the control channels are transmitted in the higher reuse, utilizing the time groups. The traffic channels are still transmitted in a ⅓ reuse.
The transmission of control information in EDGE Compact is different than the control channel transmissions in present GSM systems. Present GSM systems have at least one carrier in each cell that transmits continuously with constant power (i.e., it transmits on all timeslots, even if there is no traffic allocated). This continuous transmission serves as a beacon in the system, for mobiles to more easily find the control channel carrier, identify the cell and e.g., make signal strength measurements for mobile assisted handover algorithms. In the EDGE Compact case, the suggested control channel carrier is discontinuous; when a certain base station transmits control information, all other base stations in that cluster that uses the same frequency are quiet.
EDGE Compact, with its discontinuous transmission on the control channel carrier frequency, provides the possibility to deploy a packet data system in a spectrum well below the 1.8 MHz, as earlier was discussed. In the example described, operators may deploy an EDGE Compact system with only three 200 kHz carriers.
It would also be interesting to provide circuit switched communication, e.g., GSM circuit switched voice communication in a system that can be deployed within a small frequency spectrum. There are, however, some fundamental differences between packet and circuit switched communication, that creates problems not addressed in the prior art. One of these difficulties relates to neighbor cell signal strength measurements.
A fundamental difference between packet switched and circuit switched communication is that for circuit switched communication, e.g., a voice call, a continuous connection in both uplink, from the mobile station to the base station, and downlink, from the base station to the mobile station, is allocated. An illustration of the allocation of up and downlink physical channels in GSM is illustrated in FIG. 2. A connection between a base station and a mobile station is allocated one timeslot for uplink traffic and one timeslot for downlink traffic. Each direction has its own spectrum allocation, i.e., uplink carriers are defined in one spectrum range and downlink carriers are defined in another spectrum range.
In FIG. 2 is illustrated that allocation of the uplink is also shifted in time from that of downlink, such that e.g., uplink timeslot 1, (UL1) is aligned with downlink timeslot 4 (DL4). The reason for this is that a mobile should not have to do both receive and transmit operations at the same time. Additionally, since uplink and downlink transmissions are separated in frequency, some time is required for mobiles to adjust transceiver filters between reception and transmission.
In the period after the receive and the transmit periods, signal strength measurements from neighbor cells may be performed. In systems employing Mobile Assisted Handover (MAHO), like e.g., the GSM system, when allocated a traffic channel, mobiles use the idle period between active slots for measurements on the control channels of adjacent base stations. Since only a few time slots are available for such measurements (there is only a 4 timeslot duration between a mobiles transmit and receive slot), the base stations are usually required to transmit continuously on all the time slots of the frequency used by the control channel. Since EDGE Compact base stations are not transmitting continuously, a mobile must know or be informed when it can measure signal strength for a certain neighbor.
With the EDGE Compact systems, it is the case that if a mobile is allocated e.g., DL2 and UL2 for traffic, then UL2 coincides in time with control channel transmissions on DL5, in the downlink. It will thus be difficult for a mobile with downlink traffic on DL2 and uplink on UL2 to make measurements on control channel on DL5. This is normally not a problem in a packet switched system, since it is rare that a mobile is allocated both uplink and downlink continuously. Additionally, it will probably be quite rare that a mobile is a sole user on a timeslot. Thus, with packet switched transmission, there will be certain GSM frames that are not busy for traffic on DL2/UL2 uplink and downlink, and the mobile can then make measurements on the control channel transmitted on downlink DL5.
However, for the case with a circuit switched connection, the situation is different. There, a connection is formed of a continuous use of both uplink and downlink. Hence, a mobile will not be able to make any neighbor cell measurements on some of the time groups where control channels are allocated on certain time slots.
It would therefore be advantageous to provide a solution where an EDGE Compact system can provide enough measurement possibilities for all the neighbor cells using different time groups, such that circuit switched communication could be introduced also in an EDGE Compact scheme.
The present invention solves the problem of providing adequate neighbor cell measurement opportunities in an EDGE Compact scheme, as described in the background description. This will enable allocation of circuit switched connections also in a system employing a control channel structure where the frequency carrying the control information is discontinuously transmitted.
In one aspect of the present invention, a slot hopping control channel is introduced. This means that a control channel is not continuously allocated on one single timeslot, but shifts use of timeslots according to some regular predefined hopping pattern. Thus, a mobile that is allocated a certain timeslot for traffic in the uplink and a corresponding timeslot in the downlink for traffic will, during the time it can make measurements, experience control channel transmissions from different base stations serving different cells.
The regular hopping pattern for the control channels ensures that an equal number of measurements can be taken for each neighbor during the time when a mobile can make measurements. The hopping sequences are made orthogonal between different base stations, such that no base station transmits control in the same timeslot on the same frequency as any other base station within a cluster.
In yet another aspect of the present invention, the slot hopping is introduced such that a certain control channel allocation always shifts to a timeslot preceding the presently allocated timeslot. For example, if a base station is transmitting control information in timeslot 5, DL 5 during a certain period, then during the next period, timeslot 3, DL 3 will be used for that control channel. By hopping backwards, there will be no overlap of uplink and downlink allocation, which would otherwise be the case by hopping in the forward direction, from timeslot 7, DL7 to timeslot 1, DL1, etc.
In yet another aspect of the present invention, the mobile is notified about the hopping sequence by reading the synchronization burst. The content of the synchronization burst does not include information about where the hopping sequence starts for a certain control channel. Instead, the allocation of a certain synchronisation burst from a certain base station, is such that it is allocated on the same timeslot as will be used for the first transmission in the following GSM frame in the predefined hopping sequence for other control channels in the same time group. The synchronization burst may or may not employ slot hopping.
In yet another aspect of the present invention, a system is described in which a slot hopping control channel is implemented. This system is able to provide measurement opportunities for mobile stations allocated continuous uplink and downlink timeslots. This enables continuous communication, such as circuit switched voice communication, in both directions in a communication system such as EDGE Compact, employing a discontinuous control channel carrier.
According to an exemplary embodiment, a radio communications system employing time reuse for transmission of at least one logical channel includes a base station and at least one mobile station. In the embodiment, the base station transmits a downlink common control or broadcast channel to the at least one mobile station on a portion of a first timeslot and on a subsequent portion of a second timeslot, wherein the second timeslot is a preceding timeslot with respect to the first timeslot. In other words, where the first and second timeslots follow a particular order of transmission, the transition from the second timeslot to the first timeslot is in reverse order as compared to the transmission order. For example, for GPRS timeslots 1 through 7, wherein control and broadcast information is typically transmitted on the odd timeslots, timeslot 1 is a preceding timeslot with respect to timeslot 3, timeslot 3 is a preceding timeslot with respect to timeslot 5, timeslot 5 is a preceding timeslot with respect to timeslot 7, and timeslot 7 is a preceding timeslot with respect to timeslot 1.
According to the invention, the portion of the first timeslot and the subsequent portion of the second timeslot can occur within a single block of four GPRS downlink frames. Alternatively, the portion of the first timeslot can occur in a first block of four GPRS downlink frames, while the subsequent portion of the second timeslot occurs in a subsequent block of four GPRS downlink frames.
In addition to transmitting on the first and second timeslots, the base station can, more generally, transmit a downlink common control or broadcast channel on successive portions of timeslots in a sequence of n timeslots (n an integer greater than 1), wherein an mth timeslot in the sequence, for every m in a range of 2 to n, is a preceding timeslot with respect to an (mxe2x88x921)th timeslot in the sequence. For example, the base station can transmit the downlink common control or downlink broadcast channel on successive portions of a sequence of n timeslots in each of a succession of GPRS multi-frames.
According to the invention, the same sequence of n timeslots can be used in every multi-frame, or the sequence of n timeslots can change from multi-frame to multi-frame. Additionally, each multi-frame can include a synchronization burst indicating which sequence of n timeslots is used in a multi-frame.
For example, the synchronization burst for a multi-frame can be transmitted in a timeslot corresponding to the first timeslot in the sequence of n timeslots used in that, or a following, multi-frame. Advantageously, the synchronization burst can be transmitted in the same timeslot in each multi-frame, or the synchronization burst can be transmitted in a timeslot in each multi-frame such that the synchronization burst is a part of a timeslot hopping pattern provided by the sequence of n timeslots in the multi-frame.
According to another exemplary embodiment, a radio communications system employing time reuse on broadcast and common control channels includes a first base station transmitting a downlink broadcast or common control channel during a first portion of a first timeslot and during a subsequent portion of a second timeslot, and a second base station transmitting a downlink broadcast or common control channel during a first portion of the second timeslot and during a subsequent portion of a third timeslot. For example, the first portion of the first time slot and the first portion of the second timeslot can occur in a first GPRS frame, and the subsequent portion of the second timeslot and the subsequent portion of the third timeslot can occur in a subsequent GPRS frame.
The exemplary system further includes a mobile station measuring the transmissions from the second base station during at least a part of the first portion of the second timeslot and measuring the transmissions from the first base station during at least a part of the subsequent portion of the second timeslot. Additionally, either of the base stations can transmit a dummy burst during a portion of the second timeslot, the dummy burst being transmitted at a power level equal to a power level used by the base station in transmitting a broadcast or common control channel on the second timeslot.